Check XpFactory equality in DrawState

BUG=skia:

Review URL: https://codereview.chromium.org/767873006
diff --git a/src/gpu/GrDrawState.cpp b/src/gpu/GrDrawState.cpp
index db70bfa..8c2d75f 100644
--- a/src/gpu/GrDrawState.cpp
+++ b/src/gpu/GrDrawState.cpp
@@ -51,6 +51,10 @@
         return false;
     }
 
+    if (!this->getXPFactory()->isEqual(*that.getXPFactory())) {
+        return false;
+    }
+
     for (int i = 0; i < this->numColorStages(); i++) {
         if (!GrFragmentStage::AreCompatible(this->getColorStage(i), that.getColorStage(i),
                                              explicitLocalCoords)) {